An automobile coating basically includes sequentially applying an electrodeposition coating film, a first base coating film (also called a water-based intermediate coating film), a second base coating film (also called a color base coating film) and a clear coating film on a steel plate as a coating material. In such a coating, there are two methods; one method includes multiple heating and curing per forming a composed coating film, and the other method includes a simultaneously curing of layered multiple coating films. In these methods, the method including a simultaneously curing of layered multiple coating films can omit a heating and curing step and provide an energy-saving coating, and is advantageous.
As a method for a simultaneously curing of layered multiple coating films, a three-coat and one-bake coating by successively forming a first base coating film, a second base coating film and a clear coating film by wet on wet coating and simultaneously heating and curing is performed. However, a conventional three-coat and one-bake coating requires a preliminary drying step (so-called a preheating step) by drying, for example, at 60 to 100° C. for 2 to 20 minutes after a first base coating composition is applied, especially in case that a water-base coating composition is used. The preheat step can prevent occurrence of mixing of two coating film layers (mixing of layers). The mixing of layers is generated when a second water-based coating film is formed on an uncured first water-based coating film immediately after the first water-based coating film is formed, and water and/or an organic solvent in an uncured second water-based coating film is (are) moved to the uncured first water-based coating film. The mixing of layers frequently deteriorates film appearance of resulting multilayer coating film.
On the other hand, further omission of a preheat step after forming an uncured first water-based base coating film has been required in view of further requirement for reduce burden on the environment such as reduction of CO2 emission and energy saving. By contrast, resulting multilayer coating film requires no less excellent film appearance than ones obtained by conventional coating method.
JP 2001-009357 A (Patent Document 1) describes a coating formation method for successively forming an intermediate coating film using a water-based intermediate coating composition, a metallic base coating film using a water-based metallic base coating composition, and a clear coating film using a clear coating composition on a substrate, characterized in that the water-based intermediate coating composition and/or the water-based metallic base coating composition comprise(s) a polycarbodiimide compound and an aqueous resin having carboxyl group (claim 1). Patent Document 1 also discloses that the method can provide formation of multilayer coating film with excellent appearance by controlling affinity and bleeding in the interlayers between respectively neighboring layers in the case where the water-based intermediate coating composition and the base coating composition are successively formed on a substrate. However, the coating formation method includes a preheat step of 80° C. for 5 minutes (paragraph [0101]).
JP 2004-358462 A (Patent Document 2) describes a method for forming a multilayer coating film characterized by sequentially applying a water-based intermediate coating composition, a water-based base coating composition and a clear coating composition on an electrodeposition coating film by wet-on-wet coating and simultaneously heating and curing them, wherein the water-based intermediate coating composition comprises an acrylic resin emulsion having a glass transition temperature of −50 to 20° C., a solid acid value of 2 to 60 mgKOH/g and a solid hydroxyl value of 10 to 120 mgKOH/g, a urethane resin emulsion having a solid acid value of 5 to 50 mgKOH/g, and a curing agent (claim 1). Patent Document 2 also discloses that the method can provide a multilayer coating film having excellent smoothness of its surface with effectively preventing mixing of layers between an intermediate coating film and a base coating film. However, the method for forming a multilayer coating film also includes a preheat step of 80° C. for 5 minutes after applying the water-based intermediate coating composition (paragraph [0117]).
JP 2009-262002 A (Patent Document 3) describes a method for forming a multilayer coating film characterized by sequentially applying a water-based intermediate coating composition, a water-based base coating composition and a clear coating composition on an electrodeposition coating film by wet-on-wet coating and simultaneously heating and curing them, wherein the water-based intermediate coating composition comprises an acrylic resin emulsion having a solid hydroxyl value of 50 to 120 and a solid acid value of 20 to 60 mgKOH/g, a completely alkyl-etherified melamine resin having alkyl side chain group with 1-4 carbon atoms, and a carbodiimide compound (claim 1). Patent Document 3 also discloses that the method can prevent mixing of layers between an intermediate coating film and a base coating film in a three-coat-one-bake coating method. However, the method for forming a multilayer coating film also includes a preheat step of 80° C. for 5 minutes (paragraph [0162]).
JP 2012-116879 A (Patent Document 4) describes a water-based intermediate coating composition containing an acrylic resin emulsion and a curing agent, wherein addition of an aqueous dispersion of dimer acid derivative to the water-based intermediate coating composition provides formation of pseudo-crystalline state in the water-based intermediate coating composition and hydrophobicity to prevent transfer of solvents including water from a water-based base coating composition. However, in the method of Patent Document 4, complete deletion of a preheat step is difficult, and mixing of layers or sagging occurs.